(Gameplay) Viegars W cooldown reduction calculation

I attempted to screenshot, but for some reason can't screenshot in the game (not tech savvy enough I guess), but feel free to check it out yourself. I had 234 stacks of phenomenal evil and items. Viegars W states that for every 50 stacks of phenomenal evil, the W gets +10% cooldown reduction. From how I read it, that means that with 234 stacks, the cooldown for the W should be reduced by 40%, but it was only reduced by 34%. I then got 252 stacks and the cooldown reduction, which should have been 50%, was 40%. This means that the cooldown reduction being applied isn't 10% like it is stated in the tooltip, but that it is a reduction of 10% of the remaining percent of the initial percent value, notwithstanding other means of cdr (potentially, because I haven't been able to verify the numbers in regards to whether it considers the initial value or the modified value after cdr from items and masteries, which I'm not certain if it calculates or interacts with those reductions correctly or how). In this case, however, the first 50 gives 10% reduction, second 50 gives 9%, third 8%, fourth 7%, and fifth 6%, adding up to 40% at 250 (numbers are rounded for ease, but the example is displayed more in depth below). This is how it appeared to apply in the games I have played. I attempted to review the tooltip on this site by going to viegar under the champions section, but that verbiage is not displayed there to review. Either the tooltip is incorrect, from what I can recall, or, at the very least, misleading (and might confuse some of the newer players, in which case it may be beneficial to modify the tooltip to prevent confusion) or the cooldown reduction from viegars W is not applying correctly. Please let me know which is the case or if it can be looked into. Thanks :)

Begin at 100% 50 stacks -10% of 100% = 90% 100 stacks -(10% of 90%=9%) = 81% 150 stacks -(10% of 81%=8.1%) = 72.9% Game display rounds to 73% remaining 200 stacks -(10% of 72.9%=7.29%) = 65.61% Game display rounds to 66% remaining 250 stacks -(10% of 65.61%=6.561%) = 59.049% Game display rounds to 60% remaining